Many types of printing apparatus are capable of printing only on a single side of the recording medium. However, the desirability of saving paper (or other types of recording medium) by printing on both sides is widely recognized. A variety of duplexing designs have previously been disclosed for reversing a side of the recording medium facing the print region after a first side has been printed, in order to allow printing on the opposite side. Duplexing units are common, not only in printers but also in other types of imaging apparatus, such as scanners.
In some low-cost printers, as described in U.S. Pat. No. 7,561,823, a duplexing unit is provided as a removable auxiliary unit that the user can decide whether or not to purchase, according to his printing needs. As disclosed in U.S. Pat. No. 7,561,823, if the duplexing unit does not include any rollers, so that the rollers in the main body of the printer provides the power to push the media through the duplexing unit, no electrical or mechanical power needs to be provided to the duplexing unit, and no mechanical moving parts are needed within the removable duplexing unit.
However, it has been found that for passive duplexing units, such as those described in U.S. Pat. No. 7,561,823 or US Patent Publication 2012/0306978, sheets of recording medium, such as photo media, that are thicker than about 0.15 mm are susceptible to binding in the duplexing unit, thereby causing paper jams. This is especially true if the wrap angle of the recording medium in the duplexing unit is greater than about 180 degrees.
What is needed is a duplexing unit for an imaging apparatus that is configured to facilitate reliable passage of recording medium through the duplexing unit without binding.